This invention concerns an invention to measure resistivity in the geological formations surrounding a well in a petroleum reservoir. More particularly, a measuring device is described, consisting of a transmitter antenna and a series of receiver antennas placed outside the lining pipe in an injection well or a production well.
During injection of water through an injection well in a petroleum reservoir it may be very useful with monitoring of the state of the reservoir. Of particular importance is to perform monitoring of the so-called oil/water contact (OWC) being the boundary surface between the usually overlying oil and the underlying water in the permeable rocks constituting the reservoir, e.g. sandstone or limestone. If water under large pressure is injected below OWC this may result in pressure increase in the oil- and gas reservoir above OWC, and result in increased outflow of oil and gas from production wells being in hydraulic communication with the injection well. A device and a method according to the invention will be used both in injection wells and production wells in order to measure and perform monitoring of the electrical properties in the reservoir to indicate, among other things, the position of the oil/water contact and its movement. It is very difficult to perform measurements of the resistivity in the geological formations if one has to measure through the wall of a metallic lining (casing) pipe, an injection pipe or production tubing. For observation of the oil/water contact""s level changes it is therefore highly uncertain to perform such observations through the wall of the wellpipe. Further, in order to care for the flow capacity, it is not convenient to arrange measuring devices inside a wellpipe during normal operation of the well.
Below will be given a simplified summary of some of the factors which affect the propagation of electromagnetic waves in a rock. According to the invention there will be emitted, from the transmitter antenna, electromagnetic waves in the form of continuous, sweep or pulsed waves. These pulses are refracted in the rock strata relatively shallowly in the geological formation so that a part of the energy is picked up in the receiver antennas.
The attenuation or reduction of the energy of the electromagnetic signal happens essentially due to three main factors:
I. geometrical, approximately spherical dispersion,
II: electrical properties (resistivity and dielectric) and
III: backscattering (homogenous backscattering from reflecting geological electrically conductive more or less homogenous horizons, and occasional backscattering due to reflecting mineral particles).
AboutI: The geometrical, approximately spherical dispersion follows approximately 1/r3 with r being the distance between transmitter and receiver.
AboutII: The electrical properties is the resistivity and the varying dielectric (called the xe2x80x9cdielectric constantxe2x80x9d.) The relative dielectric constant varies from 6 for 20%-porous oil saturated quartz sand to ca. 13 for water saturated 20%-porous quartz sand. The resistivity in the rocks also determines the attenuation of the electromagnetic pulses. In FIG. 5 the attenuation in dB/m is displayed as function of frequencies between 1 and 16 MHz, for resistivities between 5 xcexa9m and 30 xcexa9m. The resistivity of oil sand in the reservoir may be between 20 and 200 xcexa9m. The resistivity of rocks containing formation water, below the oil/water contact (OWC) is between 0.5 and 5 xcexa9m. Thus the electromagnetic pulses will be attenuated much more while the transmitter- or receiver antennas are situated below the oil/water contact OWC than while the receiver antennas find themselves surrounded by oil-saturated sandstone.
AboutIII: Backscattering or reflection occurs from geological or fluid surfaces being homogenously continuous at an extent of comparable with the wavelength of the electromagnetic waves. For the actual rocks this is for wavelengths between 2 and 8 MHz crudely estimated from 10 to 2 meters. In this invention""s connection this reflection a pure loss of signal.
AboutIII: Occasional backscattering happens especially by point reflection from electrically conductive mineral grains in the rocks, e.g. pyrite, haematite and magnetite.
Examples of the Known Art.
An apparatus for measurement of formation resistivity through casing pipes is given in U.S. Pat. No. 5,680,049. The U.S. patent has electrodes being pressed against the casing pipe from a logging sonde by means of hydraulics. The logging thus occurs through an electrically conducting casing pipe which will mask the much lower conductivity (i.e. higher resistivity) in the rocks outside the casing pipe.
One method for more direct measurement of formation resistivity and reservoir monitoring outside a casing pipe is given in U.S. Pat. No. 5,642,051. Electrodes are cemented fixed in the well outside the casing pipe in hydraulically isolated zones of the reservoir. A current is sent between an electrode in the ground outside the reservoir and the electrodes in the well. In column 2 in U.S. Pat. No. 5,642,051 is described that an electrical isolation is required on the outside of the casing pipe. In this way one may regard the method for less actual for most purposes, as one must take into account that smaller and larger rifts in the isolation around the casing pipe must be expected during the installment, especially for petroleum wells below the seabed. It is also very difficult to imagine communication with electrodes in a production or injection well by means of electrically conducting in the seabed, as the wellstring from a floating platform will be impossible to isolate electrically.
An electromagnetic pulse transmitter is described in U.S. Pat. No. 4,849,699. In the actual U.S. patent the pulser is designed into a logging tool which by definition is arranged to be displaced through a borehole or a lined well.
Another pulse induction logging tool is described in U.S. Pat. No. 4,481,472.
One aspect of the present invention is
Solution to the problem.
A device and a method to measure resistivity in the geological formations outside a wellpipe, e.g. in the form of a production-, injection- or casing tube is given by the following patent claims defining this invention. The invention is a device for measurement and monitoring of resistivity in a petroleum reservoir in a geological formation with an injection-, observation- or production well comprising an electrically conductive metallic wellpipe, with the new and inventive being characterized by the following features:
a) an electrically energy source;
b) at least one inductive transmitter antenna for electromagnetic waves, fixedly arranged in the well, outside the wellpipe""s outer metallic surface;
c) at least one sensor series comprising n inductive or possibly magnetostrictive or electrostrictive resistivity sensors (5a, 5b, . . . , 5n) arranged for receiving the electromagnetic waves and generation of measurement signals, fixedly arranged in the well, by the petroleum reservoir and outside of the wellpipe""s outer metallic surface; and
d) a signal conductor for the measurement signals.
The invention also comprises a method for measurement of resistivity in a petroleum reservoir in a geological formation with an observation- or production well by means of a device being defined above, characterized in that it comprises the following steps:
i) emission of electromagnetic waves from the inductive transmitter antenna to preferably the upper part of the petroleum reservoir,
ii) reception of electromagnetic waves by a sensor series consisting of n inductive or possibly magnetostrictive or electrostrictive resistivity sensors (5a, 5b, . . . , 5n) arranged by the petroleum reservoir;
iii) generation of signals representing the electromagnetic waves sensed by the resistivity sensors (5a, 5b, . . . , 5n); and
iv) registration of signal representations of the signals.
Additional features by the invention is given in the dependent claims.